Control system for electroluminescent image-retaining panel



July 8, 1969 V P. w. RANBY ET AL 3,454,830

CONTROL SYSTEM FOR ELECTROLUMINESCENT IMAGE-RETAINING PANEL Filed Nov.2, 1964 Sheet of 2 Li l w PETER W HlT TEN RANBY INVENTORS BY W Kan-ATTORNEY MALCOLM OWEN NORRIS y 8, 1969 P. w. RANBY ET AL 3,454,830

CONTROL SYSTEM FOR ELECTROLUMINESCENT IMAGE-RETAINING PANEL Filed Nov.2, 1964 Sheet 2 of 2 /RP W PETER WHITTE'N iRANBY 'INVENTORS MALCOLM OWENNORRIS M mm.

ATTORNEY United States Patent "cc 3,454,830 CONTROL SYSTEM FORELECTROLUMINESCENT IMAGE-RETAINING PANEL Peter Whitten Ranby and MalcolmOwen Norris, London, England, assignors to Thorn Electrical IndustriesLimited, London, England, a British company Filed Nov. 2, 1964, Ser. No.408,253 Claims priority, application Great Britain, Nov. 18, 1963,45,505/ 63 Int. Cl. G05f 1/10; H05b 37/02, 39/04 US. Cl. 315-311 6Claims ABSTRACT OF THE DISCLOSURE An image-retaining electroluminescentdevice having electrodes with a phosphor embedded in a ceramic dielectric therebetween is found to work more ellectively with a highervoltage in the dark, that is, with no image, than is required to retainthe image afterward. As a consequence, a high dark voltage can be usedif voltage after irradiation is reduced to prevent deterioration of thedevice. The higher dark voltage secures quicker response, but must bebelow the voltage necessary to produce illumination in the absence ofirradiation. Means is provided to allow a high dark voltage, with areduction to a lower irradiated voltage. The means can be a fixedresistance, a resistance with a positive temperature coefiicient, or oneor more electron tubes, such as pentodes. The latter is the mostelfective means.

The present invention relates to electroluminescent devices andconstitutes an improvement in or modification of the invention claimedin British Patent N0. 966,730.

In the earlier application referred to there is claimed anelectroluminescent device comprising a layer of phosphor embedded in aceramic material disposed between two electrodes at least one of whichis transparent, and means for maintaining a unidirectional potentialdifference between the electrodes, the nature of the phosphor andceramic layer and the magnitude of the unidirectional potentialdifference being such that when the unidirectional potential differenceis applied between the electrodes the layer emits substantially nolight, but upon excitation of the layer by radiation or cathode rays orhigh energy nuclear particles which cause it to emit light, it continuesto emit light for a substantial time while the said potential differenceis maintained.

Devices such as are set out in the preceding paragraph usualy consist ofa metal plate, preferably of iron or iron-containing metal, to whichhave been applied one or more vitreous enamel coatings, one of whichincorporates a luminescent material or phosphor and an electricallyconducting film which is substantially transparent to light andinsulated from the metal base plate.

The devices with which the present invention is concerned may beconstructed in any of the ways described in the earlier specificationreferred to.

The essential feature of devices such as have been referred to above isthat if a DC. potential difiFerence be applied between the electrodes inthe dark, no light is emitted from the panel, but if the panel isexposed to light or to a wide range of other electromagnetic radiationsinvluding X-rays and the emission from certain lasers, or to cathoderays or to high energy nuclear par- 3,454,830- Patented July 8, 1969ticles, then after the exposure and so long as the direct potentialdifierence is maintained between the electrodes, the panel emits lightover that portion of its surface which has been exposed to the radiationor other bombardment above mentioned, and this emission of lightcontinues for a substantial time, for instance 20 to 30 minutes or evenup to an hour or more. However, if the potential difference is removed,the emission of light from the surface is discontinued and when a directpotential difference is again applied between the electrodes, again nolight is emitted until the phosphor layer has been exposed once more tosome form of radiation or other bombardment as above mentioned.

Panels of this kind are useful for a number of purposes, for example forstoring light patterns or messages, for use with optical lasers insetting up the option of the system, for obtaining X-ray pictures whichcan be studied after the X-ray source has been switched off, forrecording electron traces as in certain recording cathode ray tubesystems, and so on.

With devices of the kind above discussed, which will be referred tohereinafter for convenience as I.R.P.s (image retaining panels), it isfound to be important that the polarity of the applied direct potentialdifference should be such that the iron or iron-containing base plateshould be negative relative to the transparent electrode. Reversal ofthe applied polarity can be useful for quickly erasing an image retainedon the panel, but the potential must be of the correct polarity when thepanel is used to retain an image or pattern.

It has been found that when I.R.P.s are connected to a source of directpotential difference in the dark, 2. small current (called the darkcurrent) flows between the electrodes. This current increases inmagnitude when the panel is exposed to exciting radiation and graduallybuilds up to a maximum. After exposure, the panel continues to emitlight where it has been exposed but the current between the electrodesgradually decreases in magnitude.

The brightness of the light emitted from an I.R.P. follows a similarcurve to the current and the current taken by the panel can therefore beused as a convenient means of measuring the behaviour of the panel. Forexample the brightness of the glowing image retained on the surface ofan I.R.P. depends on the current, so that if the voltage applied to thepanel is increased then the current taken by the panel is increased andso the brightness of the glowing image is increased. However, there is alimit to the voltage which can be applied to the panel without causingelectrical breakdown. Moreover the maximum voltage which can be appliedto the panel when not excited by radiation (and consequently in acondition of low conductivity) is different from the maximum voltagewhich can he applied without causing breakdown when the panel isirradiated and therefore in a more highly conducting state.

We have discovered that there are considerable advantages in operatingthese panels in such a manner that the applied direct voltage can bevaried. For example if an I.R.P. be operated at a given voltage duringthe excitation by radiation in order to obtain a visible image, thenafter the source of excitation is removed the applied voltage canadvantageously be increased so that the brightness of the image isincreased. If the panel had been operated at the higher voltage while itwas being irradiated, the higher conductivity at this stage might havecaused either overheating or electrical breakdown.

Furthermore we have found that the unexcited (that is the unirradiated)panel can have a higher direct voltage applied to it than that normallyapplied for satisfactory operation during excitation or when emittinglight. Now the speed of response of the panel to radiation, i.e. thesensitivity of the panel, is greater at the higher applied voltage, butif this voltage were to be applied continuously after excitation thenthe panel might overheat or be overloaded. It is important to make useof this property in order to increase the speed of response of thesepanels, and one of the objects of the invention is to provide meanswhereby this can be done.

According to the present invention there is provided anelectroluminescent device comprising a layer of phosphor embedded in aceramic material disposed between two electrodes at least one of whichis transparent, and means for maintaining a unidirectional potentialdifference between the electrodes, the nature of the phosphor andceramic layer and the magnitude of the unidirectional potentialdifference being such that when the unidirectional potential differenceis applied between the electrodes the layer emits substantially nolight, but upon excitation of the layer by radiation or cathode rays orhigh energy nuclear particles which cause it to emit light, it continuesto emit light for a substantial time while the said potential differenceis maintained, wherein means are provided for varying the saidunidirectional potential difference as a function of the reciprocal ofthe current flowing between said electrodes.

Further according to the present invention there is provided anelectroluminescent device comprising a layer of phosphor embedded in aceramic material disposed between two electrodes at least one of whichis transparent, and means for maintaining a unidirectional potentialdifference between the electrodes, the nature of the phosphor andceramic layer and the magnitude of the unidirectional potentialdifference being such that when the unidirectional potential differenceis applied between the electrodes the layer emits substantially nolight, but upon excitation of the layer by radiation or cathode rays orhigh energy nuclear particles which cause it to emit light, it continuesto emit light for a substantial time while the said potential differenceis maintained, wherein the circuit between the electrodes including thesaid means for maintaining a unidirectional potential difference betweenthe electrodes includes series-connected resistive means.

The invention will be described by way of example with reference to theaccompanying drawings, which show circuit diagrams of a number ofdifferent embodiments of the invention.

Referring to FIG. 1, there is shown a circuit including an I.R.P., asource of unidirectional potential difference in the form of a battery Band a series resistor R. When the panel is excited and when thus thecurrent increases, an increasing proportion of the applied voltage fromB is dropped across the resistor R. By proper selection of the value ofthe resistor R the variation in the voltage across the I.R.P. can bemade suitable.

As shown in FIG. 2 the resistor R of FIG. 1 may be replaced by a devicesuch as a filament lamp L whose resistance increases with the currenttherethrough. The filament of a normal incandescent electric lamp has aresistance which increases many times as its current rises from zero toits normal working value.

A pentode or a beam power tetrode valve has a characteristic relatingits anode current to its anode voltage whose slope approximates closelyto that required, and such a valve can be incorporated as shown at Vwith an I.R.P. as indicated in FIG. 3. The cathode heater terminals areindicated at H--H. This circuit has the ad vantage over one containing aresistor that the value of the limiting current can be adjusted byadjusting the control grid voltage using for example a potential dividerP in parallel with a source of direct potential difference B Even bettercontrol can be obtained as shown in FIG. 4 by using two pentodes V and V(or two beam power tetrodes or a combination of the two). Suitableconnections for these valves are indicated in FIG. 4 and do not requirefurther explanation. The two valves may of course be incorporated withina single envelope.

In FIG. 5 there is shown a circuit arrangement including a triode Vconnected in series with a pentode V (01' a beam power tetrode) and inthis way both the voltage and the current are readily controlled. Inthis case also the valves can of course be combined in a singleenvelope.

As indicated in FIG. 6 the resistor R of FIG. 1 may be arranged to benormally short-circuited by means of a fuse F, this fuse being of suchvalue that it will blow when the panel is in its maximum conductingstate.

As indicated in FIG. 7 the fuse may be replaced by a relay S which maybe arranged to be manually operated by means of a. switch SW.Alternatively as shown in FIG. 8 the relay S may be arranged to beoperated automatically with the aid of a triode valve V The triode V maybe replaced by a thyratron valve.

Instead of using batteries to provide the control grid voltage on thevalve in FIG. 3 a variable resistor R may be connected in series withthe cathode, as shown in FIG. 9. Circuits similar to those of FIGS. 4and 5 but incorporating this series cathode resistor are shown in FIGS.10 and 11 respectively.

It will be understood that only some of the possible ways by which thedesired characteristics can be obtained have been described. Equivalentresults can be obtained by using other types of resistors or circuitsequivalent to resistors.

We claim:

1. An electroluminescent device comprising a layer including phosphormaterial embedded in ceramic material, an electrode on each side of saidlayer, means for maintaining a unidirectional potential differencebetween said electrodes, the nature of the phosphor and ceramic layerand the magnitude of the unidirectional potential difference being suchthat when the unidirectional potential difference is applied between theelectrodes the layer emits substantially no light, but upon excitationof the layer by radiation or cathode rays or high energy nuclearparticles which cause it to emit light it continues to emit light for asubstantial time while the said potential difference is maintained, andmeans for varying said unidirectional potential difference as a functionof the reciprocal of the current flowing between said electrodes.

2. An electroluminescent device comprising a layer including phosphormaterial embedded in ceramic material, an electrode on each side of saidlayer, means for maintaining a unidirectional potential differencebetween said electrodes, the nature of the phosphor and ceramic layerand the magnitude of the unidirectional potential difference being suchthat when the unidirectional potential difference is applied between theelectrodes the layer emits substantially no light, but upon excitationof the layer by radiation or cathode rays or high energy nuclearparticles which cause it to emit light it continues to emit light for asubstantial time while the said potential difference is maintained, acircuit connecting said electrodes, and a resistive means connected insaid circuit in series with said means for maintaining theunidirectional potental difference.

3. A device according to claim 2, wherein said resistive means includesthe anode-cathode path of an electron discharge tube.

4. A device according to claim 2, comprising manually operable means forvarying the resistance of said resistive means.

5. A device according to claim 2, wherein said resistive means comprisescurrent-sensitive resistive means, the resistance of said resistivemeans increasing with increase in the current therethrough.

6. An electroluminescent device comprising a layer including phosphormaterial embedded in ceramic material, an electrode on each side of saidlayer, means for maintaining a unidirectional potential dilferencebetween said electrodes, the nature of the phosphor and ceramic layerand the magnitude of the unidirectional potential ditference being suchthat when the unidirectional potential difference is applied between theelectrodes the layer emits substantially no light, but upon excitationof the layer by radiation or cathode rays or high energy nuclearparticles which cause it to emit light it continues to emit light for asubstantial time while the said potential difference is maintained, acircuit connecting said electrodes, a resistive means connected in saidcircuit in series with said means means responsive to current flowingbetween said electrodes to vary automatically the resistance of saidresistive means.

References Cited UNITED STATES PATENTS 2,985,795 5/1961 Bird 3l5307 X3,215,847 11/1965 Ranby et al. 3l5-l50 2,149,080 2/1939 Wollf 32322 10JAMES W. LAWRENCE, Primary Examiner.

PALMER C. DEMEO, Assistant Examiner.

US. Cl. X.R.

for maintaining the unidirectional potential difference, and 15 313-408;3 l5-150, 307

